The present invention relates generally to semiconductor devices and more specifically to fuse circuits employed in semiconductor devices.
The design concept of making redundant cells or circuits to replace defective cells or cells to be reconfigured is of vital application in the fabrication of integrated circuits. Defective cells on a chip which arose in the fabrication process are usually identified using a chip probe. The defective cells can then be replaced with redundant cells by the redefinition of connective paths. With such a reworking process to correct fabrication defects, the number of defective cells can be greatly reduced. The use of redundant cells is employed in most memory chips and logic circuit chips to increase the yield and reduce production costs.
There are several ways of modifying the connections to substitute redundant cells for defective cells. The use of a fuse circuit is one of the most widely employed methods for swapping defective cells with redundant cells. Typically, fuse circuits are incorporated in the integrated circuit design, and the fuses of the fuse circuits are selectively blown, for example, by passing an electrical current of sufficient magnitude to cause them to open. Frequently, fuses are blown by the use of laser. In such a case, a fuse window is normally formed through insulating layers above the fuse in the area where the fuse will be blown so that the laser heating will be more effective. The laser is focused through the fuse window, the fuse absorbs the heat from the laser irradiation and the fuse melts.
Typically, logic circuits are repaired or reconfigured by blowing fuses. For example, it is common to initially fabricate a generic logic chip having a large number of interconnected logic gates. Then in a final processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry. Still other applications of laser-blown fuses are possible.
Unfortunately, conventional semiconductor devices with fuse circuits have several drawbacks. One of the problems with these fuse circuits is that they do not contribute to the ordinary operation of the device. Further, they require silicon area that could otherwise be used for circuits that perform other functions. The space under a fuse window is normally unused. This lost space is significant considering the scaling down of features sizes in the sub-micron region and the fact that at least several hundred fuse circuits are required for each device. Further still, conventional semiconductor devices with fuse circuits do not adequately discharge the charge from an ESD during laser processing.
For these reasons and other reasons that will become apparent upon reading the following detailed description, there is a need to avoid using much or all of the silicon area that is occupied by fuse circuits, which silicon area could otherwise be used for circuits that perform other functions.